The present invention relates notably to tubular structures, in particular the tubular structures intended to transport hydrocarbons used offshore. Such tubings are typically submarine pipes. The offshore operation of an oil or gas deposit located at great depth is generally done from a floating production system situated on the surface, for example from an FPSO (Floating Production Storage and Offloading) or from a floating oil platform. Now, these floating systems are not static and move under the effect of the currents, the swell, the waves and the wind. Consequently, the submarine pipes used to raise the hydrocarbons from the seabed to the surface have to be capable of being deformed to accommodate the movements of the floating system.
These pipes are therefore designed to withstand the bends and the repeated bend variations that they will have to undergo in service for a long time (20 years and more). In the absence of measurement, the bends are estimated by computation, which means assuming significant safety margins and therefore pointlessly over-dimensioning the pipes. This is why it is desirable to be able to measure the bend of a pipe in service, preferentially in the areas most subject to fatigue stress, which is the aim of the present invention.
In the case of a riser linking the seabed to a floating support, the most critical area is generally situated in the upper part, in proximity to the connection with the floating support. In effect, at this point, the pipe undergoes, on the one hand, a very strong tension linked to its suspended weight (commonly, several hundreds of tons), and, on the other hand, strong bend variations linked to the movements of the floating support. These combined static and dynamic loadings can generate a pipe fatigue phenomenon at this point. Depending on the configuration of the pipe, there may be other critical areas, notably in proximity to the point of contact between the pipe and the seabed (touch-down point). The critical areas are generally submarine and inaccessible because of a hostile environment, which complicates the bend measurement problem.
The document WO2009/109745 describes the use of an inclinometer fixed onto the pipe and a positioning system fixed onto the floating support. The document describes the determination of relative inclination of the pipe relative to the floating support. However, this document does not provide any detailed information on the inclination measurement, or on how to determine the bend from the measurements performed.